Humanity has now become as powerful a geological agent in shaping the operation of the planet as the oceans, ice sheets and rivers, to the extent that many believe we have entered a new geological era. What is happening to the planet? How confident are we that we understand the changes, and how should we respond to them if the science is uncertain? These matters have important economic, social and philosophical implications, and present unique political problems (the recent flooding is a small-scale example). How should we respond?